1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer which is formed on the electroconductive support and contains a specific resin. In addition, the present invention relates to an electrophotographic image forming apparatus and method using the above-mentioned photoconductor, and a process cartridge including the photoconductor, which process cartridge is freely attachable to the image forming apparatus and detachable therefrom. The present invention also relates to a long-chain alkyl group containing bisphenol compound and a polymer made from the bisphenol compound, which is useful when used in an electrophotographic photoconductor.
2. Discussion of Background
To achieve image formation by electrophotography, the surface of an electrophotographic photoconductor (hereinafter referred to as a photoconductor) is uniformly charged in the dark, for example, by corona charging, and exposed to light images to selectively dissipate electric charge of a light-exposed portion, thereby forming latent electrostatic images on the surface of the photoconductor. The latent electrostatic images are developed as visible toner images with a toner that is made up of a coloring agent, such as a dye or pigment, and a polymeric material. The toner images formed on the photoconductor are transferred to an image receiving member and fixed thereon. After the toner images are transferred to the image receiving member, residual toner on the surface of the photoconductor is removed therefrom, and the photoconductor is subjected to a quenching step. Image formation can thus be repeated, using the photoconductor, by the so-called Carlson process, for an extended period of time.
Photoconductive material for use in the above-mentioned photoconductor is roughly divided into an inorganic photoconductive material and an organic photoconductive material.
Most of the currently available photoconductors employ organic photoconductive materials. This is because an organic photoconductive material is superior to an inorganic material in terms of the degree of freedom in selection of wavelength of light to which the photoconductive material is sensitive, the filming forming properties, flexibility, transparency of the obtained film, mass productivity, toxicity, and cost.
The photoconductor repeatedly used in the electrophotographic process or the like is required to have basic electrostatic properties such as good sensitivity, sufficient charging potential, charge retention properties, stable charging characteristics, minimal residual potential, and excellent spectral sensitivity. In addition to the above, the photoconductor is also required to have satisfactory physical properties from the viewpoints of printing resistance, wear resistance, and moisture resistance.
In recent years, data processors employing the electrophotographic process have exhibited remarkable development. The image quality and printing reliability have noticeably improved, in particular, in the field of a printer that adapts a digital recording system by which information is converted into a digital signal and recorded by means of light. Such a digital recording system is applied to not only printers, but also to copying machines. Namely, a digital copying machine has been actively developed. Further, there is a tendency for the digital copying machine to be provided with various data processing functions, so that demand for the digital copying machine is expected to rise sharply.
A function-separation layered photoconductor has become the mainstream in the field of electrophotographic photoconductors for the above-mentioned digital copying machine. The function-separation layered photoconductor is constructed in such a manner that a charge generation layer is provided on an electroconductive support directly or via an undercoat layer, and a charge transport layer is further overlaid on the charge generation layer. To improve the durability of the photoconductor from the mechanical and chemical viewpoints, a protective layer may be overlaid on the top surface of the photoconductive layer.
When the surface of the function-separation layered photoconductor is charged and thereafter exposed to light images, the light passes through the charge transport layer and is absorbed by a charge generation material for use in the charge generation layer. Upon absorbing light, the charge generation material produces a charge carrier. The charge carrier is injected into the charge transport layer and travels along an electric field generated by the charging step to neutralize the surface charge of the photoconductor. As a result, latent electrostatic images are formed on the surface of the photoconductor.
In view of the above-mentioned mechanism of the function-separation layered photoconductor, a charge generation material which exhibits absorption peaks within the range from the near infrared region to the visible light region is often used in combination with a charge transport material that does not hinder the charge generation material from absorbing light, in other words, exhibiting absorption within the range from the visible light region (yellow light region) to the ultraviolet region.
As a light source capable of coping with the above-mentioned digital recording system, a semiconductor laser diode (LD) and a light emitting diode (LED), which are compact, inexpensive, and highly reliable, are widely employed. The LD most commonly used these days has an oscillation wavelength range in the near infrared region of around 780 to 800 nm. The emitting wavelength of the typical LED is located at 740 nm.
The beam spot size of the LD or LED is in the range of about 60 to 150 μm. Therefore, the resolution obtained by currently available electrophotographic image forming apparatus is about 300 to 600 dpi at most, which is not sufficient to produce a high-resolution image equivalent to a photograph. To narrow down the beam spot size to about 30 μm to increase the resolution to 1200 dpi, or to about 15 μm to increase the resolution to as high as 2400 dpi, extra optical parts of extremely high precision as well as bulky optical members become necessary. In light of cost and space in the apparatus, such an electrophotographic image forming apparatus has not been put to practical use. Therefore, to produce images with a higher resolution to the extent stated above, shortening of the emitting wavelength of the employed light source has been considered effective. For instance, Japanese Laid-Open Patent Application 5-19598-discloses an electrophotographic image forming apparatus employing a laser beam with a shorter wavelength.
Recently, an LD or LED with oscillation wavelengths of 400 to 450 nm to emit a violet or blue light has been developed and finally put on the market as a light source for writing information so as to cope with the digital recording system. This kind of LD or LED is hereinafter referred to as “shorter wavelength LD or LED.” In the case where a shorter wavelength LD, of which the oscillation wavelength is as short as nearly half the conventional one located in the near infrared light region, is used as the light source for writing, it is theoretically possible to decrease the spot size of a laser beam projected on the surface of a photoconductor, in accordance with the following formula (A):d∞(π/4)(λf/D)  (A)wherein d is the spot size projected on the surface of the photoconductor, λ is the wavelength of the laser beam, f is the focal length of a fθ lens, and D is the lens diameter.
Further, from the use of such a shorter wavelength LD or LED it will be possible to make the electrophotographic image forming apparatus compact as a whole, and to speed up the electrophotographic image forming method. Accordingly, there is an increasing demand for high sensitivity and high stability of the electrophotographic photoconductor so as to cope with the light source of the LD or LED having wavelengths of about 400 to 450 nm.
As previously mentioned, the function-separation layered photoconductor has been the mainstream of the electrophotographic photoconductors. With such a layered structure, the charge transport layer is usually overlaid on the charge generation layer. High sensitivity can be obtained if light emitted from the shorter wavelength LD or LED can efficiently reach the charge generation layer after passing through the charge transport layer. Namely, it becomes important that the charge transport layer not absorb the light from the LD or LED.
The charge transport layer is generally a film with a thickness of about 10 to 30 μm made from a solid solution in which a low-molecular weight charge transport material is dispersed in a binder resin. Most of the currently available photoconductors employ as a binder resin for the charge transport layer a bisphenol polycarbonate resin or a copolymer of a monomer of the above-mentioned polycarbonate resin and any other monomers. According to the spectroscopic analysis, the bisphenol polycarbonate resin has the characteristics that no absorption appears in the wavelength range from 390 to 460 nm. Therefore, the bisphenol polycarbonate resin does not severely hinder the light for a recording operation from being transmitted through the charge transport layer.
The following are commercially available charge transport materials that are conventionally known: 1,1-bis(p-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene (Japanese Laid-Open Patent Application 62-30255), 5-[4-(N,N-di-p-tolylamino)benzylidene]-5H-dibenzo[a,b]cyclo-heptene (Japanese Laid-Open Patent Application 63-225660), and pyrene-1-aldehyde 1,1-diphenylhydrazone (Japanese Laid-Open Patent Application 58-159536). These conventional charge transport materials exhibit absorption in the wavelength range of 390 to 460 nm. Therefore, the light-emitted from the above-mentioned shorter wavelength LD or LED is unfavorably absorbed in a surface portion of the charge transport layer. As a result, the light cannot reach the charge generation layer, whereby the photosensitivity cannot be obtained in principle.
Japanese Laid-Open Patent Applications 55-67778 and 9-190054 state that when light with a particular wavelength which will be absorbed by the charge transport material is used, a decrease in charging characteristics and an increase in residual potential are caused during repeated operations. Light absorption by the charge transport material lowers the photosensitivity, and in addition, has an adverse effect on the fatigue behavior in the repetition.
Japanese Laid-Open Patent Application 9-240051 discloses an electrophotographic image forming apparatus which employs as a light source an LD beam with an oscillation wavelength of 400 to 500 nm. An electrophotographic photoconductor for use in the above-mentioned image forming apparatus is constructed in such a manner that a charge transport layer and a charge generation layer are successively overlaid on an electroconductive support in that order to aim at high resolution of the obtained image. However, the charge generation layer in the form of a fragile thin film is exposed to mechanical and chemical hazards in the cycle of charging, development, image transfer, and cleaning steps. The photoconductor deteriorates too badly to be used in practice.
The above-mentioned Japanese Laid-Open Patent Application 9-240051 also discloses an electrophotographic photoconductor of a single-layered structure. This kind of photoconductor has the problems that design of the constituent materials is limited and the sensitivity cannot increase as high as that of the function-separation layered photoconductor.
In the field of the electrophotographic image forming apparatus such as printers and copying machines, the diameter of a photoconductor tends to decrease in line with the development of high-speed operation, small-size apparatus, and high-quality image formation. This tendency makes the operating conditions of the photoconductor much more severe in the electrophotographic process.
For example, a charging roller and a cleaning rubber blade are disposed around the photoconductor. An increase in hardness of the rubber and an increase in contact pressure of the rubber blade with the photoconductor become unavoidable to obtain adequate cleaning performance. As a result, the photoconductor suffers from wear, and therefore, the potential and the sensitivity of the photoconductor are always subject to variation. Such variation produces abnormal images, impairs the color balance of color images, and lowers the color reproducibility.
In addition, when the photoconductor is operated for a long period of time, ozone generated in the course of the charging step oxidizes a binder resin and a charge transport material. Further, ionic compounds such as nitric acid ion, sulfuric acid ion, ammonium ion, and organic acid compound ion generated in the charging step are accumulated on the surface of the photoconductor, which will lead to great deterioration of image quality.
In light of the above, it is considered important to upgrade the durability of the photoconductor and improve the properties of the top surface layer of the photoconductor.
As means for solving the problem of deterioration of image quality, addition of a fluorine-containing resin such as polytetrafluoroethylene and a silicone resin such as polydimethylsiloxane to the photoconductive layer is proposed to decrease the surface energy of the photoconductor. This proposal aims to improve the durability of the photoconductor and to reduce the amount of ionic compounds deposited on the surface layer of the photoconductor.
For instance, the top surface layer of a photoconductor disclosed in Japanese Laid-Open Patent Application 4-368953 comprises finely-divided particles of a fluorine-containing resin. The top surface layer of a photoconductor disclosed in Japanese Laid-Open Patent Application 5-113670 comprises as a binder resin a siloxane-copolymerized polycarbonate resin to provide the top surface layer with lubricating properties. Namely, this proposal aims to improve the cleaning characteristics and to prevent moisture-absorption materials such as a toner and paper dust from being deposited in the form of a film on the surface layer of the photoconductor.
Furthermore, many trials have been made to prevent a decrease in image quality by providing a protective layer on the surface of the photoconductor.
For example, a protective layer comprising a variety of resins and fillers such as silica gel and tin oxide is provided on the surface of the photocondutor to improve the wear resistance of the photoconductor (Japanese Laid-Open Patent Applications 57-30843, 1-205171, 3-155558, 7-333881, 8-15887, 8-123053, 8-146641, and 8-179542.) Further, Japanese Patent Publication 5-046940 proposes the provision of a surface protective layer comprising a crosslinked polysiloxane made from a trifunctional alkoxysilane and a tetrafunctional alkoxysilane through hydrolysis and condensation.
However, the solubility of the fluorine-containing resin such as polytetrafluoroethylene in general-purpose solvents is very poor, so that it is difficult to achieve optically uniform dispersion. In addition, when such a fluorine-containing resin is added to any other resins, the fluorine-containing resin causes aggregation because of poor compatibility with other resins, whereby light scattering is induced. Further, the fluorine-containing resin tends to cause bleeding when added to any other resins.
When polysiloxane is added to other resins, the bleeding also occurs, with the result that the effect by the addition of the polysiloxane does not last for long. Furthermore, a polysiloxane is a polymer provided with electrical insulating properties, so that the charge transporting properties of the photoconductor are hindered by the polysiloxane when the protective layer contains a polysiloxane.
When the protective layer is prepared using a resin in which a filler is dispersed, the surface energy generally increases to impair the cleaning characteristics although the surface hardness of the photoconductor can improve. Further, the filler particles tend to aggregate in the protective layer to cause light scattering.
In addition to the above-mentioned problems, the potential of a light portion on the photoconductor tends to increase while the photoconductor is continuously used for an extended period of time. The result is that image quality is caused to deteriorate because of a decrease in image density and a decrease in image resolution.